Part of maintaining rigor is acknowledging situations where professional judgments don’t agree, and finding ways to come to an understanding. Sometimes people will look at a side-by-side comparison and come to opposite conclusions. (...) The first step is to have a conversation about the source of the disagreement. Very often it turns out that different professionals have different criteria for success for a visualization, or have different goals in mind; clarifying these is extremely useful to the field. Other times, however, people simply have different intuitions about clarity or legibility. In these situations, it may make sense to turn to a scientific experiment. This should not be viewed as a failure of criticism, but rather a success: a crisp, testable scientific question is a rare commodity

And here's another one, which is key:

The field of visualization sits at the intersection of two very different intellectual traditions. On one side of the family, visualization traces its roots to art and graphic design. On the other side, it’s descended from computer graphics and the tradition of scientific experiment. It’s worth taking a step back and describing some of the morés and norms in each field, and how they conflict in the case of visualization criticism.

The three recommendations at the end are great, too: maintain rigor, respect the designer, and respect the critic.